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Photons get the quantum cloning treatment

Mar 28, 2002

Near-perfect copies of single photons have been made in the lab for the first time. Quantum systems cannot be cloned – or duplicated – perfectly, but the development of quantum cryptography and computing relies on a knowledge of exactly how well they can be copied. Antia Lamas-Linares and co-workers at the University of Oxford sent a photon into a crystal where it stimulated the emission of another photon with almost the same properties, confirming theoretical predictions (A Lamas-Linares et al 2002 Science to appear).

Conventional computers store information as ‘bits’, which can have a value of either 1 or 0. As electronic components become smaller, physicists have suggested that information could be stored in certain two-level quantum systems. These include the horizontal and vertical polarization states of photons, or the ‘spin up’ and ‘spin down’ states of electrons. But the crucial difference is that these quantum bits – or ‘qubits’ – can exist in both possible states at the same time, a phenomenon known as superposition.

Many conventional computing tasks rely on ‘parallel processing’, in which bits are duplicated and operated upon simultaneously to solve a problem more quickly. But the state of a quantum system can never be fully known, so perfect duplication of qubits is forbidden. This is the ‘no-cloning’ theorem, which is the basis of quantum cryptography.

Lamas-Linares and co-workers based their experiment on two photons created by the ‘down-conversion’ – or splitting – of a single higher-frequency photon. These photons are linked – or ‘entangled’ – so that a measurement of the polarization of one reveals the polarization of the other.

The Oxford team sent a photon from one such pair into an optically active crystal where it stimulated the emission of a further photon. There is an increased chance that the new photon will have the same polarization as the ‘input’ photon. In contrast, photons that are emitted spontaneously are equally likely to be in either polarization state.

Since the input photon was one of an entangled pair, Lamas-Linares and colleagues were able to compare the polarizations of the new photon and the second photon created in the down-conversion step – and therefore with the polarization of the original photon. They believe that the wave qualities of the new photon overlapped with those of the original photon by the maximum possible amount calculated by theorists – that is, five sixths.

“This is the first time that cloning has been demonstrated for individual quantum systems,“ Lamas-Linares told PhysicsWeb. “The use of stimulated emission is a particularly natural choice.”

In earlier experiments, physicists at Oxford cloned large assemblies of quantum systems (arXiv.org/abs/quant-ph/0111098), and a group at the University of Science and Technology of China imprinted the polarization and the motion of a single photon onto two clones (Phys. Rev. A 64 012315). But Mark Hillery of Hunter College of the City University of New York believes that the work by Lamas-Linares and colleagues is the best demonstration yet, because it produced copies of the highest theoretical quality. “It is nice to see cloning moving off paper and into the laboratory,” he told PhysicsWeb.